1. Field of the Invention
The present invention relates to particulate size reduction, and more particularly, to journal bearing assemblies for pulverizers.
2. Description of Related Art
A variety of devices and methods are known in the art for reducing particulate size of raw materials. Of such devices, many are directed to pulverizing coal for use as a fuel. In coal-fired furnaces, for purposes of improved and more efficient ignition, it is preferred to pulverize the coal to a fine powder before introducing it into the furnace for combustion.
In operations that use coal for fuel, finely-ground coal particles or “fines” are desirable for efficient operation, yielding higher combustion efficiency than stoker firing, as well as rapid response to load changes. Using coal fines for combustion also produces less nitrous oxide (NOX) emissions and keeps oversized loss-on-ignition (LOI) unburned coal particles from contaminating the marketable ash byproduct of the combustion chamber. Thus, it is common practice to supply raw coal to a device, such as a pulverizer, that will reduce the size of the coal to particles within a desirable range prior to being used for combustion.
Coal pulverization involves systematically comminuting coal to a desired size, e.g., a fine powder, prior to introduction into a coal-fired furnace. Conventional coal pulverization systems include ball tube type mills, high-speed attrition type pulverizers, and vertical roller type mills.
Traditional bowl mill pulverizers are shown and described in U.S. Pat. No. 3,465,971 to Dalenberg, et al., and U.S. Pat. No. 4,002,299 to Skalka, which describe the construction and operation bowl mills suitable for use in coal fired power generation systems. As taught by the patents above, a typical bowl mill includes a body portion in which a grinding table is mounted for rotation, a plurality of grinding rollers that roll against the grinding table to grind coal therebetween, coal supply means for feeding raw coal to the interior of the bowl mill, and air supply means for supplying an air flow to the interior of the bowl mill, which entrains airborne pulverized coal particles for combustion.
In such a bowl mill, the coal enters the bowl mill and is pulverized between the grinding rollers and the grinding table. After being pulverized, the coal particles are thrown outwardly by centrifugal force whereby the particles are fed into a stream of air that is entering the bowl mill. The stream of air, which now contains pulverized coal particles, flows through a tortuous path that is established in part by the positioning of a suitably supported deflector means within the bowl mill. As the stream of air and coal particles flows along the aforementioned tortuous path, the sharp turns effect the separation of the coarse coal particles from the air stream. These coarse coal particles are then returned to the grinding table for further pulverization, while the fine coal particles are carried out of the bowl mill in the air stream. The capacity of each bowl mill of the type described above can be on the order of 100 tons per hour of coal. As the industry has pressed for greater and greater capacity, the size of typical bowl mills has increased in an effort to meet the demand.
With increased size have come issues related to stress and wear. More specifically, prolonged operation of conventional of bowl mills can only last as long as the service life of the weakest components. The pulverizer journal systems typically employed in conventional bowl mills have suffered from one or more of the following undesirable features. It has been found that the upper bearing has been subjected to high radial loading. It has also been found that the lower bearing has been subjected to high thrust loading. Furthermore, it has often been found that the upper bearing does not have the capacity desirable for the loadings encountered during ordinary service.
The result of these factors is that the bearings are often require replacement earlier than the other major components of a typical journal assembly. In order to replace the journal bearings, downtime is required. Thus it is often the case that downtime is incurred due to the bearings per se. The more frequently the need arises to replace the journal bearings, the more the resulting downtime adds up in the long run. Thus, there is a significant need for improvements in the service life of bearing support for journals on which pulverizer rolls are mounted.
The roller of a typical journal assembly is supported by a pair of spaced apart journal bearings. Each journal assembly has a loading point that is typically defined as the intersection point of the longitudinal axis of the journal assembly and the line on which the resulting load acts on the roller. The conventional assemblies were designed to accommodate a hypothetical loading point within the span between the upper and lower bearings. However, in practice, the loading point tends to actually lie outside the span between the two bearings, above the upper journal bearing. If the actual loading point were in fact located at the hypothetical loading point for which the system was designed, radial and thrust loading would have been ameliorated. The actual loading point location results in the undesirable loading factors described above.
The discrepancy between the hypothetical loading point and the loading point in practice is due at least in part to the assumptions made in determining the hypothetical loading point. Earlier assemblies were smaller than more recent assemblies. Assumptions used to determine the loading point that may have been reasonable for smaller assemblies may not be reasonable for determining the loading point of larger assemblies. Roller size, table velocity, and other factors that have increased over the years tend to undermine the original assumptions. Also, due to the smaller size in the earlier assemblies, some error in loading point position could be tolerated with less noticeable effects on service life.
One solution has been to essentially enlarge the upper bearing as described in U.S. Pat. No. 4,538,768 to Paskowski, Jr. et al., which describes using a double tapered roller bearing for the upper journal bearing in place of single tapered roller bearing. Using a larger bearing, or double bearing, can increase the service life of the upper and lower bearings and thus the whole journal assembly. But the cost of the upper bearing is significantly increased. The loading point in this double tapered roller bearing design also falls outside the span between upper and lower journal bearings in practice, despite the hypothetical loading point shown in the Paskowski Patent.
While the conventional methods and systems have generally been considered satisfactory for their intended purposes, there still remains an ongoing need in the art for improved support of journal assemblies that allow for longer service life and less down time. There also remains a need for such methods and apparatus that are easy to make and use. The present invention provides a solution for these problems.